HE Jiajun ¹, ZHANG Wanbin ², YANG Fan ³, ZHAO Lin ¹, XUE Ju ¹,

CAO Didi ¹, WEI Hongkang ¹, YU Meiling ¹

(1. School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China;

2. School of Mechanical Engineering, Jiangxi Vocational College of Industry & Engineering, Pingxiang 337000, Jiangxi, China;

3. Guangxin Testing and Certification Group Co., Ltd., Jinan 250002, Shandong, China)

Extended abstract:[Background and purposes] The ultra-high temperature ceramic matrix composites (UHTCMC), prepared by reinforcing and toughening ultra-high temperature ceramics (UHTCs) with fibers as the matrix, can be used for service in extreme environments, due to their excellent mechanical properties, thermal stability, oxidation resistance and ablation resistance. Zirconia carbide ceramic matrix composites (ZrC-CMC) have become one of the most attractive UHTCMC materials. Compared with chemical vapor infiltration (CVI), precursor infiltration and pyrolysis (PIP) and reactive melt infiltration (RMI), the slurry infiltration (SI) method is relatively simple, inexpensive and widely used to prepare high-density ZrC-CMC with promising mechanical properties and oxidation resistance. The preparation of spherical ZrC nanopowders is of great significance in obtaining high-quality ZrC-CMC by using the SI process. As compared with solid phase carbothermal reduction (CTR) and solid phase discharge plasma sintering (SPS), sol-gel is effective in producing nanopowders at relatively low temperatures, but it is also difficult to control the morphology. Therefore, preparation of ZrC nanopowders with controllable morphologies remains a major challenge. In this paper, ZrC precursors were first synthesized with tetrabutyl zirconate and acetylacetone as raw materials, and then ZrC nanopowders were prepared through calcination. Molecular group, viscosity, phase composition and micro-morphology of the ZrC precursors and nanopowders were characterized and analyzed.[Methods] Acetylacetone and tetrabutyl zirconate were stirred magnetically at 110 ℃ for 72 h to synthesize precursors with molar ratios of 4.5:1.0, 5.0:1.0, 5.2:1.0, 5.4:1.0, 5.5:1.0, which were denoted as ZrC-4.5, ZrC-5.0, ZrC-5.2, ZrC-5.4, ZrC-5.5, respectively. Then, the as-synthesized ZrC precursors were pretreated at 800 ℃ for 3 h in Ar, at a heating rate of 5 ℃·min−1. Finally, the samples were p sintered by using spark plasma sintering in vacuum at 1200–1600 ℃ for 0–2 h. Rotational viscometer (HB DV-Ⅱ+pro) was used to measure the viscosities of the as-synthesized liquid precursors. Fourier transform infrared (FTIR) spectrum of the precursor was obtained with Bruker Vertex70 spectrometer. Thermo-gravimetric analysis (STA 449 F3) was used to investigate the thermal decomposition behavior of the precursors. X-ray diffraction (D/MAX2500), Scanning electron microscopy (JSM-6700F) and Transmission electron microscopy (JEM 2100F) were used to analyze the phase composition, microstructure and element distribution of the ZrC nanopowders, respectively.[Results] As shown in infrared spectrum of the ZrC precursor, the vibration of C-H causes absorption peaks at 3070 cm⁻¹, 2959 cm⁻¹, 1073 cm⁻¹ and 752 cm⁻¹, while the absorption peaks at 1728 cm⁻¹, 1528 cm⁻¹ and 1428 cm⁻¹ are assigned to the vibration of C=O. The C-C vibration is related to the absorption peaks at 1179 cm^-1, 1101 cm⁻¹ and 1024 cm⁻¹, while the absorption peak at 1600 cm⁻¹ is attributed to the C=C vibration. The deformation of C₃O₂Zr ring and Zr-O vibration result in absorption peaks at 713 cm⁻¹, 580 cm⁻¹ and 418 cm⁻¹, indicating that acetylacetone acts as chelating ligand. The as-synthesized ZrC precursors have linear structure and exist in liquid state, due to the steric hindrance during the reaction. The precursors are transparent brownish yellow liquid, with the color being gradually deepened with increasing molar ratio of acetylacetone and tetrabutyl zirconate, due to the light absorption effect of acetylacetone coordinated with zirconium atoms. As the content of acetylacetone increases, the viscosity of the ZrC precursors gradually increases, which may be related to the increase of free acetylacetone content in ZrC precursor. However, the maximum viscosity is only 15.31 mPa·s. As revealed by the thermo-gravimetry results, the entire mass loss process has three main stages, with the total mass loss to be in the range of 33–47%. The release of free and non-coordinating molecules and the decomposition of precursors, are the main causes of mass loss in the first stage (up to 240 ℃). The mass loss in the second stage (240–560 ℃) is caused by further decomposition of some carbon hydroxide compounds. The carbon thermal reduction reaction at 1200–1400 ℃ is the main cause of the third stage mass loss. The mass losses of ZrC-4.5, ZrC-5.0 and ZrC-5.5 at 1600 ℃ are 33.0 wt.%, 41.5 wt.% and 46.8 wt.%, respectively. As shown in XRD patterns, ZrC-5.5 exhibits diffraction peaks of tetragonal zirconia (t-ZrO₂) after being calcining at 800 ℃, indicating that the ZrC precursor begins to decompose into ZrO₂. As the temperature is increased to 1400 ℃, t-ZrO₂ undergoes carbon thermal reduction reaction with the carbon source generated by the decomposition of acetylacetone to form ZrC. When the temperature reaches 1600 ℃, the crystallinity of ZrC increases significantly. According to Scherrer equation, the grain size of ZrC nanopowder is about 32 nm. Diffraction peaks of ZrC, t-ZrO₂ and monoclinic zirconia (m-ZrO₂) can be observed in ZrC-4.5 powder after sintering at 1600 ℃ for 2 h. As the molar ratio gradually increases to 5.2, the coexistence of ZrC and m-ZrO₂ can still be observed. When the molar ratio reaches 5.5, only pure ZrC phase exists, due to carbon thermal reduction. When ZrC-5.5 is calcined at 1600 ℃ for 0 h, tetragonal and monoclinic zirconia are present. While the diffraction peaks of tetragonal and monoclinic zirconia phases disappear completely and pure ZrC nanopowders are obtained after calcining for 0.5 h. As shown in SEM images, the particle size increases and the crystallinity improves gradually with increasing calcination temperature and holding time. The grain size is still relatively small (<100 nm) after calcinting at 1600 ℃ for 2 h. It can be observed from the TEM images, the as-prepared ZrC nanopowders are elliptical particles with average grain size of <30 nm. Meanwhile, the particle size and morphology of the powder are relatively uniform.[Conclusions] The as-synthesized ZrC precursors have linear structure and exist in liquid state, due to the steric hindrance during the reaction process of acetylacetone and tetrabutyl zirconate. The color of the ZrC precursors darken and the viscosity increases gradually as the molar ratio increases, but the maximum viscosity is only 15.31 mPa·s. The mass loss of ZrC precursors is divided into three stages with mass losses of 33.0 wt.%, 41.5 wt.% and 46.8 wt.% for ZrC-4.5, ZrC-5.0 and ZrC-5.5 after calcining at 1600 ℃, respectively. When the molar ratio of acetylacetone to tetrabutyl zirconate is 5.5:1, pure ZrC nanopowders with particle size of 30 nm can be obtained after calcining at 1600 ℃ for 2 h. The ZrC precursor first forms t-ZrO₂ and then gradually transforms into t-ZrO₂ and ZrC with increasing calcination temperature. Moreover, the particle size and crystallinity increase gradually. The as-prepared ZrC nanopowders are elliptical, grain size is <30 nm and the morphology is uniform.

Key words: ZrC; precursors; sintering; nanopowders; ultra-high temperature ceramics